Electroluminescent displays have phosphor elements which emit light when they are electrically energized. Typically, electroluminescent displays have a glass substrate on which a layered structure of electrically conducting electrodes and phosphor is disposed. The phosphor emits light when an energization signal is applied to the electrodes of the display. The light emitted by the phosphor is observed through the glass substrate, which is the front screen of the display.
It has been found that the phosphor and electrode elements of the display can be covered by a back cap which may be made of metal or glass. The back cap encloses the phosphor and electrode elements and is adhered to the glass substrate, for example by epoxy. The space defined within the back cap can include a dessicant, such as alumino silicate beads which absorb water and thereby increase the life of the phosphor of the display.
If the interior of the sealed back cap includes air or gas under atmospheric pressure, for example 15 psi, the relatively rigid metal or glass material of the back cap is well-suited to provide a near hermetic seal if the external ambient pressure is similar to the pressure in the back cap. However, if the back cap is exposed to a relatively low ambient pressure, for example as would occur if the display is shipped by air in an unpressurized container, the pressure differential between the interior of the back cap and the surrounding air will cause the back cap, glass substrate or both of them to rupture and therefore break the hermetic seal. Exposure of the phosphor of the panel to ambient air and moisture will result in a rapid and severe degradation of the panel and loss of useful life for the display.
It would be possible to avoid rupturing the back cap and substrate by increasing the thickness of the material of these elements. However, the required thickness of material for pressure differentials encountered in air transportation would result in a panel which is undesirably heavy. Also, a sturdy metal back cap could expand and contract relative to the glass substrate in response to changes in ambient temperature and could therefore break the seal.
Moreover, if the internal pressure of the back cap is reduced significantly below ambient pressure, the back cap and substrate will tend to implode. Thus, a substantial pressure differential resulting from lower or higher external pressure can result in breaking of the hermetic seal and destruction of the panel.
It is desirable to provide a relatively light back cap which will hermetically seal the phosphor and electrode elements of an electroluminescent display, despite changes in ambient pressure.
Accordingly, the basic object of the invention is to produce a display with a compliant back cap which can be evacuated and which will not be affected by fluctuations of ambient air pressure.
A further object of the invention is to provide such an electroluminescent display with a back cap which is made of at least one compliant metal layer.
Another object of the invention is to provide such a compliant back cap which contains a dessicant in vacuum and which presses the dessicant against the phosphor of the display, so that ambient air pressure is equally maintained on the back cap and underlying phosphor.
A further object of the invention is to provide a method for making a compliant back cap of an electroluminescent display.
These and other objects of the invention will become apparent from a review of the drawings and the detailed description which follows.